1. Field of the Invention
The present invention relates to a sheet conveying apparatus which conveys a sheet and an image forming apparatus which forms an image in the sheet.
2. Description of the Related Art
Conventionally, there is known a switchback configuration in which a sheet conveying direction is reversed by switchback. For example, Japanese Patent Application Laid-Open No. 2002-205861 discloses a configuration in which the sheet is guided using a rotatably-supported switching guide plate in performing the switchback conveyance. In the configuration disclosed in Japanese Patent Application Laid-Open No. 2002-205861, the sheet to be conveyed is guided by one surface of the switching guide plate before the switchback is performed, and the sheet is guided to a reverse conveying path by the other surface of the switching guide plate after the switchback. Japanese Patent Application Laid-Open No. 11-310356 discloses a configuration in which the switching guide plate for switching the sheet conveying path is controlled by an actuator.
In the configurations in which the switching guide plate is used as disclosed in Japanese Patent Application Laid-Open Nos. 2002-205861 and 11-310356, after a sheet rear end passes through an end portion of the switching guide plate, the switching guide plate is moved such that the other surface of the switching guide plate guides the sheet, and the switchback of the sheet is performed to introduce the sheet to the reverse conveying path, which results in a long conveying distance for performing the switchback and an enlarged apparatus. In the case where the switching guide plate is controlled using the actuator, the apparatus becomes further enlarged and complicated. Therefore, as disclosed in Japanese Patent Application Laid-Open Nos. 2-66053, 2004-292150, and 2-75564, there is a configuration in which the sheet conveying path is shortened to perform the switchback using a pair of rollers and not the switching guide plate.
The configuration in which the switchback is performed using the pair of rollers will be described with reference to FIG. 10. FIG. 10 is a sectional view illustrating a schematic configuration of a laser beam printer 99 (hereinafter referred to as “apparatus”) which is of an example of an image forming apparatus including a conventional sheet conveying apparatus. The apparatus 99 can perform the duplex image formation of the sheet S. The apparatus 99 includes an image forming portion 91, a fixing portion 93, and a sheet reverse conveying portion 92 which is of the sheet conveying apparatus for reversing the sheet to convey the sheet to the image forming portion again.
In the case where the duplex image formation of the sheet S is performed by the apparatus 99, the image formation is performed on one surface of the sheet S by the image forming portion 91, and the sheet S is delivered to the fixing portion 93 to fix the image. Then, the sheet S is conveyed to a pair of reversing rollers 924. The pair of reversing rollers 924 are temporarily stopped when the rear end of the sheet S passes through a first nip N1 between a fixing and discharge roller 921 and a first roller 922. The rear end of the sheet S in which the image formation is completed on one surface is released from constraint between the fixing and discharge roller 921 and the first roller 922, and the sheet S is stopped along an outside of the conveying path by a nip angle of the pair of reversing rollers 924. Then, the reversal rotation of the pair of reversing rollers 924 performs the reverse conveyance of the sheet S to a second nip N2 between the fixing and discharge roller 921 and a second roller 923. Then, the sheet S is conveyed to the image forming portion 91 again by the conveying roller pair 926, and the sheet S is discharged from a discharge roller pair 925 through the fixing portion 93 after the image formation is performed to a backside of the sheet S.
At this point, in order to deal with the case in which the end portion of the sheet S is curled toward the image forming surface side, Japanese Patent No. 3058720 discloses a configuration in which a guide shape is provided to smoothly convey the sheet S to the second nip N2. Japanese Patent Application Laid-Open No. 11-035210 discloses a configuration in which a guide is provided to guide the sheet, reverse after temporarily conveyed into the sheet discharge path, to a return path for conveying the sheet to the image forming portion again. The guide can be moved between a position where the guide is retracted from a sheet discharge path and a position where a front-end portion of the guide protrudes into the sheet discharge path. The sheet is guided to the return path by the guide located at the position where the front-end portion of the guide intrudes in the sheet discharge path.
It is thought that the sheet is curled due to heat and pressure imparted by the fixing portion 93 when passing through the fixing portion 93. In the case where the end portion of the sheet S is curled toward the image forming surface side, sometimes the end portion of the sheet S is headed to the first nip N1, the fixing and discharge roller 921, or the first roller 922 when the sheet S should be conveyed to the second nip N2 by the reversal rotation of the pair of reversing rollers 924. Because the first nip N1 is rotated in a direction in which the sheet S is discharged at that time, conveyance failure is generated to cause damage to the end portion of the sheet. The sheet whose end portion is curled cannot surely be prevented from intruding in the first nip N1 by the guide shape disclosed in Japanese Patent No. 3058720 and the guide, disclosed in Japanese Patent Application Laid-Open No. 11-035210, which is located at the position where the guide intrudes in the sheet discharge path.